High-purity silicon for manufacture of semiconductor elements and solar cells is generally manufactured in a multi-step operation proceeding from metallurgical silicon, which generally includes a relatively high proportion of impurities. An example of a possible way to purify metallurgical silicon is to convert it into a trihalosilane such as trichlorosilane (SiHCl3), which is subsequently subjected to thermal decomposition into high-purity silicon. A procedure of this type is, for example, known from DE 29 19 086 A1.
A further development of this process is due to Union Carbide Corp. and described in DE 33 11 650 A1. Trichlorosilane is not immediately decomposed in this process. Instead, it is subjected to a disproportionation reaction yielding monosilane (SiH4) and tetrachlorosilane (SiCl4) as end products. The monosilane obtained is then thermally decomposed in lieu of trichlorosilane. Advantageously, the decomposition products formed are nearly exclusively hydrogen (H2) and metallic silicon. By contrast, the decomposition of trichlorosilane leads to highly corrosive compounds such as hydrogen chloride (HCl).
The disproportionation of chlorosilanes such as trichlorosilane may be catalyzed. Catalysts that have proved to be particularly advantageous include basic catalysts such as, for example, the amine compounds known from DE 25 07 864 A1. These are preferably employed in bound form, as described, for example, in DE 33 11 650 A1. Catalysts bound to solid carriers are simple to separate from liquid or gaseous reaction mixtures. This is why the virtually exclusive use of amine catalysts—either fixed to carriers or embedded in crosslinked polymers—is state of the art in the industrial disproportionation of chlorosilanes.
It is known, inter alia from DE 198 60 146 A1, to perform the disproportionation of trichlorosilane according to the principle of reactive distillation. Reactive distillation combines reaction and distillation, specifically rectificative separation in one column. Trichlorosilane may be disproportionated over a suitable catalyst within that column while at the same time low-boiling products resulting from the disproportionation reaction are removed from the column by distillation, specifically rectification. The yield of a disproportionation reaction carried out in a closed reaction vessel is limited as a chemical equilibrium becomes established. Reactive distillation, in contrast, by ensuring the ongoing removal of low-boiling products, results in a continuous displacement of the equilibrium, enhancing the yield of the disproportionation reaction and the efficiency of the overall operation.
DE 100 17 168 A1 discloses accommodating within one column a plurality of reactive/distillative regions of reaction in an arrangement where they are stacked on top of each other. A condenser arranged between the regions separates a monosilane-containing product mixture from comparatively high-boiling chlorosilanes. That form of process variant is relatively demanding in terms of equipment requirements, but does ensure that comparatively high-lying regions of reaction are not excessively affected by high-boiling chlorosilanes.
DE 10 2009 032 833 A1 discloses a monosilane disproportionation reaction column accommodating two reactive/distillative regions of reaction one on top of the other. The two regions of reaction are operated at different temperatures and also contain solids having different catalytic effects and thermal stabilities in that the solids chosen for the lower region of reaction are of higher thermal stability than contemplated for the upper region of reaction. A column heated via the column sump only can accordingly be operated at comparatively high temperatures since less care and attention has to be devoted to the thermal stability of the catalyst in the lower region of reaction. The rate of disproportionation can be raised in this way.
It could therefore be helpful to improve existing technical solutions for disproportionation of chlorosilanes, specifically trichlorosilane and provide for high energy efficiency and high yields and be realizable in a plant of very simple equipment design.